Page 321 - Design of Simple and Robust Process Plants

8.3 Automation of Operation 307
. Condition irreversible process units close to operational conditions; this is
often realized by setting up a recycle system or a dummy load, as applied for
furnaces.
. Take irreversible process systems in operation to achieve quality product
from the start by designed trajectory control.
When this procedure is applied, the three processes referred to during the previous
start-up discussion of these plants are as follows:
. For the hydration process (Figure 8.8), the start-up takes the following
sequential steps:
± The utility systems are activated.
± The last two reversible separations are taken in operation in reversible mode.
± The reversible separation after the reactor section is activated.
± The irreversible reactor is conditioned by putting the recycle from the separa-
tion as being one of the reactants, back to the reactor. Bring the reactor at
start-up conditions with respect to temperature and pressure through a
recycle.
± Start the reactor according to designed trajectories for reactants flow and tem-
perature.
. For the HDA process (Figure 8.9), start-up takes the following sequential
steps:
± The utility system are activated
± The boiler is filled with boiler water and pressurized from the steam grid; the
boiler unit is now at standby conditions. Strictly speaking, this is not a revers-
ible unit at this point in time ± it is only conditioned. It enters a reversible
mode when the recycle compressor loop is active with the heater.
± The reversible separations B/T and T/DP at the back end of the process are
taken in operation.
± The reversible compressor unit is put in re-circulation mode, together with
the H 2 and CH 4 separations.
± The initial heat source in the reactor system (an irreversible furnace opera-
tion) is activated; this is required to initiate the reactions. The boiler starts to
produce steam, while the reactor system with its heat train is now condition-
ing up to its operational level.
± Start-up of the reactor system with feed-through implementation of the de-
signed trajectories for flow rates, temperatures and with H 2 quench.
. The start-up of the nitric acid plant (Figure 8.10) has the following sequence:
± The utility systems are activated.
± Accommodation of the steam reboiler by filling it with boiler water and pres-
surizing with steam from the grid (conditioning).
± Fill the absorber with an initial amount of water; the trays are equipped with
bubble caps with a high weir height to create sufficient residence time for
reaction. A recycle over the bottom part. with acid is provided.
± Start-up of the air compressor in open recycle mode over the atmosphere while
bringing the combustor and the reactive absorber to open recycle condition.